Color dichroic polarizers and method for the production thereof

ABSTRACT

In order to produce full-color dichroic polarizers, it is necessary to produce absorption bands with several absorption maxima. The absorption bands produced until now always had a simple band form which was determined by the form of the non-spherical particles deposited in the polarization layer.  
     By briefly warming a partial layer of a layer that is provided with not totally relaxed non-spherical silver particles, an adjustment of a given axis ratio occurs only in this partial layer, while absorption bands with several absorption maxima can be produced by means of differently adjusted axis ratios in superimposed partial layers. Full-color polarizers can be produced in this manner by combining several such absorption bands in a pixel-form structure.

[0001] The invention relates to color, preferably multicolor, dichroic polarizers and a method for their production. In this type of polarizers the color- and polarizing effect is produced by the dichroic absorption of non-spherical particles, chiefly metallic particles, deposited in glass as a substrate material. The invention relates to the forms of the particles in the substrate material and thus to the form and spectral site of the absorption bands, as well as to a method for the adjustment of the same.

[0002] The field of use of the invention is polarizers in the visible, ultraviolet and near infrared spectral region preferably with dichroic absorption bands laterally differently adjusted in a specific manner. Such polarizers are suitable, i.a., for the production of displays.

[0003] It is known that uniformly oriented non-spherical particles in substrate materials can lead to dichroic absorption bands. Typical examples are silver, copper, or gold particles in glasses. Oriented spheroid metallic silver particles lead, e.g., in glasses to dichroic absorption bands in the visible, ultraviolet and near infrared spectral region. In the visible spectral region, the dichroic absorption causes a color effect that is dependent on the polarizing direction. In the case of silver particles, it is characteristic thereby that the dichroic behavior is produced by a single absorption band. The absorption band in the visible spectral region can be placed in principle at different points of the visible spectrum, by means of which various color effects can be adjusted. The site of the absorption band maximum in the spectrum is essentially determined thereby by the form of the particles. In the case of ellipsoid particles, the spectral site of the absorption band is determined by the semiaxis ratio of the particles.

[0004] There are numerous suggestions that utilize this effect for special applications.

[0005] A method for the production of a dichroic polarizer for liquid crystal displays is known from DE 29 27 230 C2 “Method for the production of a polarized glass film, glass film produced accordingly, and use of such a film in liquid crystal displays”. The starting point is an organic or inorganic glass melt into which needle-shaped bodies are introduced and from which a glass film is drawn.

[0006] It is known to produce highly polarizing glasses on the basis of phase-separated silver halide-containing glasses in which silver halide particles of the desired size are produced by tempering (U.S. Pat. No. 3,653,863). This is followed by two further steps: First the glass is stretched, extruded, or rolled at temperatures between the upper cooling point and the glass transition temperature in order to endow the silver halide particles with an ellipsoid shape and to orient them in the same direction. Then the glass is exposed to a radiation, e.g., UV radiation. Metallic silver is deposited on the surface of the silver halide particles thereby. These glasses can be adjusted between clear unpolarized and darkened-polarizing, by irradiation.

[0007] Furthermore, it is known to temper glass below the cooling point in a reducing atmosphere in order to produce elongated silver particles in a surface layer of the glass of at least 10 μm thickness (U.S. Pat. No. 4,304,584). The production of a glass combined to create a laminate, whereby polarizing and photochromic glass layers are combined and laminated, is described there.

[0008] It is known to laminate a metal halide-containing glass with another glass before the deformation process, in order to achieve higher eccentricities of the metal particles (U.S. Pat. No. 4,486,213).

[0009] It is known to produce UV polarizers in that the formation of metal particles occurs in a surface layer of glasses by means of a repeated change in the introduction of metal ions and tempering (DE 198 29 970). The result of this is the formation of spherical particles with a certain size distribution. With a subsequent deformation of the glass, spheroid particles of differing size with differing semiaxis ratios are formed.

[0010] These methods have in common that submicroscopic, as a rule spherical, foreign phase particles are produced in a substrate matrix, which particles are subsequently deformed in a deformation process and are oriented uniformly in a preferred direction. The resulting dichroic absorption bands of the deformed foreign phase particles are essentially determined by their form and are thus fixed.

[0011] Furthermore, it is known that when the substrates are warmed to temperatures near to or above the transformation temperature of the glass, a relaxation of the particles back to the spherical shape occurs. A change in the dichroic absorption is associated with this. Depending on the temperature and duration of their treatment, particles with any desired degree of relaxation between the starting condition and spherical form can be produced. However, the tempering process, whose duration is usually in the range of hours, allows only the monochromatic adjustment of dichroic polarizing glasses to certain colors.

[0012] It is known to shorten the relaxation times for the transition of the metal particles from one semiaxis ratio to a smaller one, down to the microsecond range, by using temperatures considerably above the transformation point of the glass (DE 196 42 116). The energy transfer occurs in a structured manner with electron beams. The method additionally allows the absorption band and thus the color effect to be adjusted differently in a specific manner in different lateral regions of the substrate. In this manner flat elements of different colors with lateral dimensions down to far below 100 μm can also be produced.

[0013] The principal deficiency of the prior art is that the absorption bands that can be produced, in the case of silver a single band, are always fixed on the band form, which is determined by the form of the particles. More complex band forms cannot be produced.

[0014] If it is desired to produce full color by additive color mixing of the primary colors red, green, and blue, however, specific demands must be made on the absorption spectra that cannot be met by the prior art. In order to obtain one of the three primary colors when illuminated by daylight, the two other primary colors must respectively be strongly absorbed. In order, for example, to obtain the colors red or blue respectively when illuminated by daylight, broad absorption bands are required that absorb in the blue and green or red and green spectral region respectively. In order to obtain green, two absorption bands with maxima in the red and blue spectral region are necessary. This lies outside the possibilities offered by the prior art.

[0015] For use of the dichroic polarizers for color displays, however, it would be necessary to arrange for small regions with such absorption bands to be closely adjacent, in order to make an additive mixture of the primary colors physiologically effective for the observer.

[0016] The object of the invention is to create a multicolor dichroic polarizer that contains closely adjacent flat regions, such as e.g. strips or pixels, in a lateral arrangement, which regions differ in the spectral site of their absorption bands and are preferably adjusted in the colors red, green, and blue. Furthermore the object of the invention is to disclose a method that allows the absorption bands required for this to be produced in dichroic polarizing glass.

[0017] It is assumed that it is possible to produce almost any desired absorption spectra by superimposing different absorption bands. The superimposition of the absorption bands can be attained by arranging partial layers with absorption bands in different spectral regions one behind the other. Parallax freedom is required for the production of desired absorption spectra on very small areas, i.e., for example, pixel structures. For this it is necessary for the partial layers lying one behind the other to be in as dense an arrangement as possible.

[0018] This dense arrangement is made possible according to the invention in that the absorbing layer is formed as at least a double layer whose two partial layers feature different absorption bands. The color dichroic polarizer constructed on this principle preferably comprises a known sodium silicate glass substrate that is provided at least partially with a double layer containing non-spherical metal particles, in particular silver particles. Their average axis ratio within one of the two partial layers of the double layer differs from the average axis ratio within the respective other of the two partial layers of the double layer. The two partial layers are thus characterized by different absorption spectra, in the case of silver particles by different single bands. The resulting absorption spectrum is thus formed from the superimposition of the absorption bands of a different spectral site of the two partial layers of the double layer lying one behind the other. In this manner colors can be displayed that can be produced only by multiband absorption spectra. If small adjacent regions are treated so that different, preferably multiband, absorption spectra form in them, the colors resulting from the different absorption spectra can be mixed additively.

[0019] In one embodiment as a multicolor dichroic polarizer, preferably three different resulting absorption spectra are achieved in this manner in different lateral regions of the double layer, in that two single bands respectively are combined or superimposed by suitable adjustment of the absorption by lateral regions of the partial layers of the double layer. These lateral regions are grouped in a regular arrangement of strips or pixels. Three of these lateral regions respectively, which by suitable adjustment of the absorption bands of the partial layers of the double layer appear in the colors red, green, and blue when illuminated with white light, are grouped to produce a flat element that can accept each such displayable color by additive color mixing. Whereas in the red and blue strips or pixels the axis ratios of the metal particles in the two partial layers of the double layer are adjusted so that they feature adjacent absorption bands lying in the blue and green or red and green region respectively, in the green regions the axis ratio of the metal particles in the two partial layers of the double layer respectively is adjusted so that they absorb in the blue or red spectral region. Thus in these lateral regions an absorption spectrum with two separate maxima in the red and blue spectral region is formed, so that only the green component of the light can transit these lateral regions of the substrate. It is only the production of such absorption spectra with absorption maxima lying far apart from one another and an adequate optical density that enables the preparation of all three additive primary colors and is a prerequisite for the full color ability of the dichroic polarizer according to the invention. Mixed colors of high saturation can be produced in particular at an optical density of the individual partial layers of ≧1 in the range of the respective absorption maxima.

[0020] The method for the production of the dichroic polarizers begins in a known manner first with the production of a surface layer in glass substrates, preferably sodium silicate glass substrates, which layer contains non-spherical metal particles, preferably silver particles, with an adequately large axis ratio in the required density and oriented arrangement. The main process steps required for this are according to prior art ion exchange, tempering, and deformation. The adjustment of lateral surface regions to certain absorption bands occurs in a known manner by quasi-adiabatic warming of the respective surface region over at least the thickness of the surface layer containing the metal particles by the appropriate action of an electron beam on the substrate.

[0021] To adjust different axis ratios of the metal particles in the surface regions that are to be formed as a double layer, according to the invention the corresponding surface regions are exposed to the brief action of an electron beam, whereby the range of the electrons in the substrate material is preferably selected by means of an appropriate accelerating voltage of the electron beam such that the temperature needed for the desired relaxation of the metal particles is achieved only in the upper partial layer of the double layer, while in the lower partial layer of the double layer the metal particles essentially remain in their initial shape. It is advisable if the relaxation of the metal particles is enabled directly in the partial layer bordering on the surface. It has proved to be particularly favorable to select the accelerating voltage of the electron beam so that the electron range in the substrate material is between two and three times the thickness of the upper partial layer of the double layer to be produced.

[0022] The invention is explained in more detail based on an exemplary embodiment and the drawings. They show:

[0023]FIG. 1 A section of a multicolor dichroic polarizer according to the invention with strip-form image elements in a perspective, greatly enlarged section representation,

[0024]FIG. 2 An illustration of the relationship between the axis ratio of the silver particles and the spectral site of the assigned absorption bands,

[0025]FIG. 3 The absorption spectra adjusted in the double layers of different lateral regions of a multicolor dichroic polarizer to display the colors red (FIG. 3a), green (FIG. 3b), and blue (FIG. 3c).

[0026]FIG. 1

[0027] The multicolor dichroic polarizer comprises a sodium silicate glass substrate 1 that contains submicroscopic silver particles 3 in a surface layer 2 embodied as a double layer. The silver particles form spheroids that are described by their semiaxis ratio. The average dimensions of the spheroids are in the range of a few 10 nm. The silver particles are oriented by the production process with their main axes parallel to one another and parallel to the substrate surface.

[0028] In partial layers 4 a and 4 b, 5 a and 5 b, 6 a and 6 b lying one behind the other respectively, the axis ratios of the silver particles are adjusted so that they feature absorption bands in the blue and green, blue and red, or green and red spectral region respectively. As a result, when the substrate 1 is transilluminated with linearly polarized white light with a polarizing direction parallel to the large semiaxis of the particles, adjacent strips 4, 5, and 6 of the substrate 1 appear in the colors red, green, or blue respectively. The thickness of the partial layers 4 a, 5 a, and 6 a bordering on the surface is the same and is fixed in accordance with the thickness of the surface layer 2 and the density- and size distribution of the particles within the surface layer 2.

[0029]FIG. 2

[0030] Non-spherical silver particles lead to a dichroic absorption effect, whereby the spectral maximum site of the absorption band is a function of the semiaxis ratio of the silver particles. A semiaxis ratio of the silver particles of a/b=3 results in a maximum site of the assigned absorption band in the red spectral region at 630 nm. With smaller axis ratios, the maxima of the corresponding absorption bands lie at smaller wavelengths. At an axis ratio close to 1, the absorption band lies in the blue spectral region at 420 nm.

[0031]FIGS. 3a-c

[0032] The absorption spectrum of the surface layer 2 is formed in the individual adjacent strips 4, 5, and 6, respectively from the superimposition of the two absorption bands of the partial layers 4 a; b, 5 a; b, and 6 a; b.

[0033] The method according to the invention for the production of the multicolor dichroic polarizer with colored image elements in the form of strips is described below. The starting point for the production of the multicolor dichroic polarizer with lateral surface regions in the form of strips in the colors red, green, and blue according to FIG. 1 is a glass substrate provided with silver particles in the entire region of the later double layer, the average semiaxis ratio of which silver particles causes an absorption band in the red spectral region. Then in a first process step, in the lateral regions 4, which are to appear red after the polarizer is finished, the substrate 1 is warmed with an electron beam by quasi-adiabatic energy transfer over the entire thickness of the surface layer 2—the later double layer—until the silver particles in these lateral regions relax over the entire thickness of the later double layer until the absorption band shifts from the red spectral region into the green spectral region.

[0034] In a second process step, with an accelerating voltage of the electron beam lower than in the first process step, only the upper regions, which when the polarizer is finished form the upper partial layers 4 a, 5 a, and 6 a of the double layer, are likewise warmed with the electron beam by quasi-adiabatic energy transfer until the absorption band in the partial layer 4 a shifts from green to blue, in the partial layer 5 a from red to blue, and in the partial layer 6 a from red to green. The energy densities transferred to the individual strips 4, 5, and 6 are thereby selected in accordance with the temperature conditions required for the respectively needed spectral shift of the absorption bands.

[0035] For a typical thickness of the double layer of 10 to 20 μm, in the first process step an accelerating voltage for the electron beam of 50 to 70 kV is used, for the second process step an accelerating voltage of <25 to 35 kV. The requirements for an adiabatic energy transfer are adequately met if this occurs for a given surface site within several microseconds up to several 10 microseconds, depending on the accelerating voltage of the electron beam.

[0036] To eliminate unavoidable slight non-uniformities, i.a., in the properties of the glass, the thickness of the surface layer and the particle distribution present therein, a process control may be necessary. The accelerating voltage of the electron beam is used thereby as an actuator to control deviations in the optical density, the transferred energy density is used as an actuator to control deviations in the spectral site of the absorption bands. 

1. Color dichroic polarizers comprising a glass substrate in which in a surface layer, non-spherical metal particles are deposited in an oriented arrangement, characterized in that in at least one lateral region of the glass substrate, the surface layer comprises a double layer whose two partial layers contain metal particles with a different average axis ratio and are thus characterized by different absorption spectra.
 2. Color dichroic polarizer according to claim 1, characterized in that the non-spherical metal particles comprise silver.
 3. Color dichroic polarizer according to claim 1 or 2, characterized in that several lateral regions with the same absorption spectrum form a regular arrangement of strips or pixels.
 4. Color dichroic polarizer according to claim 3, characterized in that next to each strip or pixel of the regular arrangement, there are two further strips or pixels that respectively feature another absorption spectrum.
 5. Color dichroic polarizer according to claims 1 through 4, characterized in that three strips or pixels are respectively grouped to produce an image element and these image elements form a regular arrangement.
 6. Color dichroic polarizer according to claim 5, characterized in that each image element contains pixels or strips in which the partial layers of the double layer have absorption maxima in the blue and green, the blue and red, and the red and green spectral region, so that when illuminated with linearly polarized white light with a polarizing direction parallel to the large semiaxis of the particles, the strips give the colors red, green, and blue.
 7. Color dichroic polarizer according to claims 1 through 6, characterized in that each of the partial layers features an optical density ≧1.
 8. Color dichroic polarizer according to claims 1 to 7, characterized in that the surface layer has a thickness of 10 to 20 μm and that all partial layers near the surface have the same thickness in the range of 5 to 10 μm.
 9. Process for the production of color dichroic polarizers from a glass substrate that contains in an oriented arrangement, non-spherical metal particles in a surface layer and at least not totally relaxed metal particles in individual lateral regions, characterized in that in partial regions of the surface layer in which non-spherical and at least not totally relaxed metal particles are deposited in an oriented arrangement, an axis ratio in a partial layer of the surface layer of these partial regions is adjusted by means of a relaxation of the non-spherical metal particles caused by brief warming, which axis ratio is smaller than in the non-spherical metal particles in the other partial layer of the partial regions of the surface layer.
 10. Process according to claim 9, characterized in that the adjustment of the smaller axis ratio occurs in the partial layer bordering on the surface.
 11. Process according to claims 9 and 10, characterized in that the warming of the partial layer of the surface layer bordering on the surface occurs by quasi-adiabatic energy transfer with an electron beam, whose depth of penetration is matched to the thickness of this partial layer via its accelerating voltage.
 12. Process according to claims 9 through 11, characterized in that the accelerating voltage of the electron beam is selected such that the electron range in the substrate material is selected between two and three times the thickness of the partial layer of the surface layer bordering on the surface.
 13. Process according to claims 9 through 12, characterized in that the energy transfer occurs in several partial amounts as a succession of pulses.
 14. Process according to claims 9 through 13, characterized in that to ensure the same optical density of the partial layers, the accelerating voltage of the electron beam is used as a regulating variable in a control process.
 15. Process according to claims 9 through 14, characterized in that to ensure the desired spectral site of the absorption bands, the transferred energy density is used as a regulating variable in a control process. 